The present invention relates to a micromechanical device, such as a micromechanical actuator or a micromechanical sensor.
Micromechanically produced and actively deflectable beam and plate structures are used for a large number and variety of applications. The deflectable plates are frequently also referred to as membranes due to their low thickness. For the actively deflectable beams, the term cantilever is frequently used. Table 1 lists some examples of microsystems and the functional principle of the actively deflectable microstructures is explained.
TABLE 1Examples of microsystems and a description ofpossible functional principles of the deflectable microstructuresFunctional principle of the activelyExample/Microsystemdeflectable microstructuresreferenceActively bendableThin mirror plates firmly clamped at the 0, [2],mirror for focusedge are bent using an electrostatic field. [3], [8],variation of lightHere, the forces from the electrostatic field [9]beams or forare effective vertically at the bottom of the specificplate.correctionThin mirror plates are bent using an0, [4],of the phase of theactively adjustable lateral material strain[5], [11]wave front of lightwithin a material layer (bimorph ormonomorph principle). The activelyamendable lateral material strain can havea thermomechanical, piezoelectric,piezomagnetic, electrostrictive ormagnetostrictive origin.Actively bendableThin annular mirror plates firmly clamped[21]focusing mirrorat the edge are bent using an electrostaticarray for trans-field. Here, the forces from themissive displayselectrostatic field are effective vertically atthe bottom of the plateActively tiltableThe mirror plates are connected to a[6]mirrors for spatialstationary frame via spring elements and[7]deviation of lightare tilted using an electrostatic field. Here,beamsthe forces from the electrostatic field areeffective vertically at the bottom of theplate. This principle is also used for microtilting mirror arrays.The mirror plates are connected to a[10], stationary frame via thin bimorph or[12],monomorph beam structures. Tilting the[13]mirror plate is performed via an activelyadjustable layer stress and the associatedbending of the beam structures. The activeamendable lateral material strain can havea thermomechanical, piezoelectric,piezomagnetic, electrostrictive ormagnetostrictive origin.Actively adjustableThe mirror plates are connected to a[15]lifting or loweringstationary frame via spring elements andmirror for theare moved orthogonally to the mirror areaphase variation ofusing an electrostatic field. Here, the forceslight (e.g. in FTIRfrom the electrostatic field are effectivespectrometers)vertically at the bottom of the plate.This principle is also used for loweringmirror arrays for adaptive optics.The mirror plates are connected to a[14]stationary frame via thin bimorph ormonomorph beam structures. Deflection ofthe mirror plate is performed via anactively adjustable layer stress and theassociated bending of the beam structures.The actively amendable lateral materialstrain can have a thermomechanical,piezoelectric, piezomagnetic,electrostrictive or magneto-strictive origin.Atomic ForceA beam (=leaf spring or cantilever) isMicroscopy deflected either via an electrostatic field(AFM) in measure-between the measurement probe and the tipment modes withof the beam or via piezoelectricallydynamic excitationinduced strain within the beam.
Basically, the functional principles of the actively deflectable microstructures presented here can be divided into two classes:
In the first class, a force acts orthogonally to the structure 100 (=beam or plate) to be deflected.
FIG. 43 exemplarily shows a micromechanical actuator of this type. FIG. 43 shows a micromechanical device having a deflectable plate or a deflectable beam 100, which is cantilevered in the present case and where an externally effected force F is effective, which brings the plate or the beam 100 perpendicular to the extension direction out of its resting position, as is indicated by an arrow 100a between the resting position on the left and the deflected position on the right in FIG. 43. Here, it should be noted that in the overall document of the present application the included drawings and sketches include schematical coordinate symbols marked by x, y and z, and that here it has been consistently taken care that the x-y plane is parallel to the substrate plane or the chip plane, in the case that the shown devices are integrated in a chip, and that accordingly the z direction is orthogonally to the substrate plane in the case that chip integration exists.
Frequently, the vertically attacking force F is the force from an electrostatic field. Here, the electrostatic field E is generated using an electric voltage U between the beam (or plate) to be deflected and a stationary electrode 101 (also referred to as counter-electrode).
FIG. 44 shows a micromechanical actuator wherein the structure 100 to be deflected is deflected in that a voltage U is applied between the structure 100 to be deflected and an electrode 101 opposing the same, which has the effect that the structure 100 to be deflected and the electrode 101 are attracted toward each other. Thus, in the case of FIG. 44, the structure to be deflected, here a cantilevered beam or plate, is deflected by the force of an electrostatic field.
The reason for the dominance of the electrostatic drive in microsystem technology is the good scaling behavior of the forces (acting on the electrodes of an electrostatic field) in the dimensions common for microsystems. For simple, plane parallel electrode assemblies performing no or only very little movement, the force is inversely proportional to the square of the electrode gap (F˜1/d2). Thus, small electrode gaps result in a high force effect on the electrodes.
The deflection w of the microstructures (beams or plates) depends in an approximately square manner on the applied electric voltage U (for plane parallel electrode assemblies F˜1/d2). The basic curve is shown in FIG. 45.
The first region of this characteristic curve is characterized by a stable equilibrium between the forces of the electrostatic field and the restoring forces of the clamp. The second region is characterized by an unstable equilibrium. A small change of the electric voltage has the effect that the forces of the electrostatic field become larger than the mechanical restoring forces of the clamp (=holder of the microstructure). The beam (or the plate) 100 thus reaches an unstable area and is accelerated up to the counter-electrode 101. This behavior is known to a person skilled in the art as “pull-in effect”:                The pull-in effect occurs in all electrostatically deflectable microstructures.        The maximum possible deflection wpull-in is limited (depending on the mechanical characteristics of the clamp) to a maximum of approx. ⅓ of the electrode gap d by the pull-in effect.        Large deflections can only be enabled by large gaps d of the electrodes. Large gaps, however, significantly increase the necessitated electric drive voltages (due to: F˜1/d2).        
Due to the pull-in effect and the ratio F˜1/d2, a large deflection of electrostatically deflectable microstructures is combined with high electric voltages. Voltages in the range of 100 V are frequently common for deflections in the μm range (e.g.: [2] and [3]).
In a second class of microstructures, a lateral force acts within one or several layers of the structure 200 to be deflected.
FIG. 46 shows a structure 200 to be deflected composed of a layer stack of two or more layers 201 and 202. A lateral strain is generated in at least one of the layers, by the force of which the structure 200 to be deflected, here again for example a beam or a plate, is deflected, as is indicated by 200a. 
This form of deflectable structures is known to a person skilled in the art as monomorph- or bimorph-deflectable cantilevers or membranes (beams or plates). The lateral force F (more accurately: lateral strain caused by the physical effect) can be caused by different physical effects:                Thermomechanical excitation (thermomechanical bimorph): Here, two materials 201 and 202 having different coefficients of linear expansion are firmly connected to one another. When this structure is heated (for example by an integrated electrothermal micro heating=usage of the resistive power), a lateral strain results and hence a lateral force of different intensities in both layers. Due to this, the microstructure is bent.        Piezoelectric and electrostrictive excitation (electroactive monomorphs, bimorphs and multimorphs using the transversal effect): Here, a lateral strain or force is generated within at least one layer 201 by an electrostatic field and by using an electroactive material. This material strain can be actively changed using the electric voltage or the electric field. As a result of this, the microstructure is bent.        Piezomagnetic and magnetostrictive excitation (magnetoactive monomorphs, bimorphs and multimorphs using the transversal effect): Here, a magnetic field and the usage of a magnetoactive material generate a lateral strain within at least one layer 201. As a result, the microstructure is bent.        
For appropriate ratios of maximum deflection w to beam or plate thickness t (w<t), a spherical deformation profile results. This results in the fact that the maximum deflection w is proportional to the square of the length l of the microstructures (w˜l2).
The advantage of the bimorph- or monomorph-deflectable microstructures is that with relatively small actively adjustable material strains large deflections result with sufficiently large structure lengths l. The maximum possible deflections are not limited (as in the above case) by the characteristics of the drive principle.
A problem associated with thermomechanical excitation is that basically sufficiently high material strains for bending micromechanical cantilevers and plates can be generated by the thermomechanical effect and a suitable material selection (for 201 and 202) [1], but the generation of the temperatures necessitated for this is problematic for three different reasons:                The height of the temperatures that can be generated with a given power coupling depends on the thermal insulation (more accurately, heat flow balance) of the microstructures to be deflected with regard to their environment. Depending on the size of the beam or plate area to be heated, high thermal insulation of the structures to be deflected might not be realized. Thus, a relatively high power consumption of the microsystem is needed for generating the temperatures necessitated for deflection.        If a resistive micro heating (=usage of ohmic “power loss”) is used, the operating range of the temperatures to be used for the target movement of the cantilever or the plate will have to be above the maximum environmental temperature of the target application. The reason for this is that a resistive micro heating can only increase temperatures. Thus, the microsystem has high power consumption even at low target deflections [5]. Basically, the temperatures could be both increased and reduced by using the Peltier effect, but due to the low efficiency a microsystem having low power consumption is not possible here either.        The amount of heat capacity (e.g. of the surrounding air) associated with the thermal insulation of the microstructures to be deflected limits the maximum possible velocity of the movement of the deflection. Due to the relatively high heat capacities, the thermal cut-off frequency is in the bottom Hz range [5], [16] even for the low dimensions of microsystems. This has the following consequences:                    If bimorph beams or plates having higher frequencies are to be moved in a quasi-static manner, a significant reduction of the deflection has to be accepted due to the low-pass behavior of temperature generation.            With step-like changes of the target deflection, the microstructure to be deflected reacts in a very slow manner (due to the low thermal cut-off frequency) and hence necessitates a lot of time until the target deflection is reached.                        
Due to the above-stated limitations (large power consumption, “constant preheating” and low cut-off frequency of deflection), the thermomechanical effect is used very rarely for actively deflecting plates or beams. A commercial microsystem using the thermomechanical effect for actively deflecting microstructures is not known on our part.
Problems accompanying excitation using electroactive or magnetoactive materials are, for example, the following. In microsystem technology, for example, primarily the usage of the inverse piezoelectric effect is common (see also [4]). A problem, however, is the usage of the common electroactive or magnetoactive materials within semiconductor-compatible production plants (frequently called “CMOS-compatible materials”): materials having a high electromechanical or magnetomechanical material coupling (such as the piezoelectric materials PZT, BaTiO3 or LiNbO3) cannot be used in semiconductor-compatible production plants or processes due to the possible contamination of the production plants or processes. Basically, there are semiconductor-compatible electroactive materials such as aluminum nitride or PVDF. However, either the electromechanical material coupling is very low (such as for aluminum nitride or gallium nitride) or these materials have no temperature or long-term stable behavior (such as zinc oxide). Optimizing the deposition conditions for high electroactive material coupling is very expensive, and in some case the materials have to be actively polarized afterwards. Magnetoactive materials basically have the problem of high power consumption of the microcomponent (similar to the thermomechanical principle), since a magnetic field having an alterable field strength has to be generated for actively deflecting the material. Generating a variable magnetic field can only be performed by a variable current flow of a coil assembly having sufficient current strength. Usage of permanent magnets is not possible here, since the same generate a “stationary magnetic field”.
In summary, it would therefore be desirable to have an alternative drive principle for a micromechanical device uniting at least some of the advantages of the different drive principles briefly sketched above, and without having to accept all the accompanying disadvantages.